An automatic voltage regulator (hereinafter, also called “AVR”) is known which keeps a voltage output from a generator winding to a previously set voltage by controlling a current supplied to a field winding by a voltage generated to an excitation winding.
An operation of an alternating-current generator having a conventional AVR (hereinafter, also called “AVR generator”) will be described referring to a drawing. In FIG. 7, a generator 100 includes a field winding 102, a generator winding 103, and an excitation winding 104. Permanent magnets 106 are disposed to a rotor 105 around which the field winding 102 is wound. A rectifier 109a, a transistor drive unit (control unit) 109, and a transistor 110 are disposed to an AVR 107. The excitation winding 104 is connected to an input side of a rectifier 108, and the field winding 102 is connected to an output side of the rectifier 108 through a brush 111. A flywheel diode 112 and a smoothing capacitor 113 are connected in parallel with the field winding 102.
When the rotor 105 is rotated by a not shown drive source such as an engine, an electro-motive force is generated to the excitation winding 104 by the permanent magnets 106 disposed on outer peripheral surfaces of the rotor 105. The electro-motive force is rectified and smoothed by the rectifier 108 as well as the flywheel diode 112 and the smoothing capacitor 113. When the transistor 110 is turned on, a current is caused to flow to the field winding 102 by the electro-motive force, and excitation is executed by a large magnetic field in addition to excitation executed by a magnetic force of the permanent magnets 106. A positive feedback is applied to a field as described above, and an output voltage of the generator winding 103 increases. The output voltage is rectified by the rectifier 109a, input to the transistor drive unit 109, and compared with a voltage target value. When the output voltage reaches the voltage target value, the transistor 110 is turned off. As a result, the output voltage of the generator winding 103 is decreased and the transistor 110 is turned on again. As described above, the transistor 110 is repeatedly turning on and off, and a constant voltage control is executed.
Generators having the AVR are disclosed in, for example, Patent Documents 1, 2, 3, 4, and the like.